Optical modulators are commonly employed in the field of optical communications to accept modulated data in electrical format (e.g., radio frequency or RF) and transfer data to an optical carrier. A Mach-Zehnder modulator (hereinafter alternatively referred to as “MZM”) generally utilizes a beam-splitter that divides laser light from an input optical waveguide into two optical beams. These optical beams then propagate in parallel waveguides defining separate optical paths, at least one of which have a phase modulator in which the refractive index is a function of the strength of the locally applied electrical field.
In today's optical networks, Mach-Zehnder modulators are widely being used as intensity and phase modulators. Initial commercially available Mach-Zehnder modulators were based on non-semiconducting materials such as lithium-niobate, but currently such MZMs are also available in a variety of semiconducting materials (in particular, III-V semiconductors) such as Indium Phosphide (InP), Gallium Arsenide (GaAs), Silicon, polymer, to name just a few.
A typical optical MZM is based on an optical splitter (e.g., multimode interference (MMI) optical waveguide or directional coupler), two optical paths (typically referred to as the “arms” of the MZM) and an optical combiner (e.g., MMI or directional coupler). An intensity or phase modulation results from the phase difference between the two optical signals propagating in each arm of the MZM. By using an electro-optic effect on one or both arms of the MZM, the refractive index of the material can be changed locally by applying a voltage, resulting in a phase shift of the optical signal propagating through such zone. As such, the MZM can be operated with a phase change in one arm or in both arms. In the latter case, the well-known term “push-pull” is used to designate the optical operation defined by causing an equal but opposite phase change in each arm, such that only half the phase change is needed on each arm compared to the case where only one arm is modulated in order to achieve the same total phase difference and resulting amplitude modulation.
For push-pull Mach-Zehnder modulators, two structures are generally utilized: so-called single-drive and dual-drive. With a single-drive MZM integrated circuit design, the problem is that only single-ended drivers can be used and that the N-region of the integrated circuit needs to be biased through a large resistor or inductor. This introduces extra cost and size, and causes certain high-pass behaviours. This high-pass behaviour results in baseline wander effect, which in in-phase quadrature (IQ) modulation, results in distorted quadrature-amplitude modulation (QAM) constellations. These types of effects cannot be easily compensated with digital signal processing, due to the low-frequency (i.e., long time period) character of such impairments.
Alternatively, in a dual-drive MZM integrated circuit design, both electrodes are ideally driven with out-of-phase signals and modulating the optical phase in both arms. In this case, the N-region is not exposed to an alternating current (AC) signal and the direct current (DC) bias can just be applied directly, without the need for a resistor or inductor. As such, this simplifies the circuit design and allows for an electro-optical transfer function that goes down to DC. Further, this dual-drive MZM configuration is also compatible with differential-output drivers which makes the design of the driver much easier and more robust. For example, in advanced silicon CMOS or SiGe BiCMOS processes, it is advantageous to make amplifiers fully-differential (i.e., differential signals in and out) for high signal frequencies. The differential structure inherently rejects such common-mode (CM) signals, which could appear due to unwanted coupling of external noise sources or because of variation on the power supply or ground nets. Moreover, differential amplifiers tend to be more linear because even-order harmonics are cancelled out, and the differential output offers double the signal swing compared to a single-ended output, for the same supply voltage. These combined advantages result in a driver that is more power-efficient and has higher performance compared to a single-ended driver.
However, one disadvantage of the dual-drive structure is that a common-mode signal on the electrodes causes a parasitic phase shift which is deleterious to MZM performance, and requires that such dual-drive MZMs be designed according to very stringent requirements to compensate for the effects of this parasitic phase shift thereby increasing design complexity and cost.
Therefore, a need exists for an improved dual-drive push-pull Mach-Zehnder modulator that mitigates parasitic phase shift effects from CM signal components.